Animal Development

Animal Development
Chapter 43
Impacts, Issues
Mind-Boggling Births
 From a single fertilized egg, all adult cells and
tissues develop – humans are learning to
manipulate the beginnings of life
43.1 Stages of
Reproduction and Development
 Animals as different as sea stars and sea otters
pass through the same stages in their
developmental journey from a single, fertilized
egg to a multicelled adult
Six Processes of
Reproduction and Development
 Gamete formation
• Egg and sperm production
 Fertilization
• Egg and sperm join to form a zygote
 Cleavage (blastula formation)
• Repeated mitotic divisions increase the number
of cells (blastomeres), not the volume
Six Processes of
Reproduction and Development
 Gastrulation
• Gastrula (early embryo) forms with two or three
germ layers (forerunners of tissues and organs)
 Organ formation
• Tissues become arranged into organs
 Growth and tissue specialization
• Continues into adulthood
Six Processes of
Reproduction and Development
a Eggs form and mature in
female reproductive organs.
Sperm form and mature in
male reproductive organs.
b A sperm penetrates an egg.
Their nuclei fuse. A zygote
has formed.
c Mitotic cell divisions form a
ball of cells, a blastula. Each
cell gets regionally different
parts of the egg cytoplasm.
d A gastrula, an early embryo
that has primary tissue layers,
forms by cell divisions, cell
migrations, and
rearrangements.
e Details of the body plan fill in
as different cell types interact
and form tissues and organs
in predictable patterns.
f Organs grow in size,
take on mature form,
and gradually assume
specialized functions.
Gamete Formation
Fertilization
Cleavage
Gastrulation
Organ Formation
Growth, Tissue
Specialization
Fig. 43-2, p. 760
a Eggs form and mature in
female reproductive organs.
Sperm form and mature in
male reproductive organs.
b A sperm penetrates an egg.
Their nuclei fuse. A zygote
has formed.
c Mitotic cell divisions form a
ball of cells, a blastula. Each
cell gets regionally different
parts of the egg cytoplasm.
d A gastrula, an early embryo
that has primary tissue layers,
forms by cell divisions, cell
migrations, and
rearrangements.
e Details of the body plan fill in
as different cell types interact
and form tissues and organs
in predictable patterns.
f Organs grow in size,
take on mature form,
and gradually assume
specialized functions.
Gamete Formation
Fertilization
Cleavage
Gastrulation
Organ Formation
Growth, Tissue
Specialization
Stepped Art
Fig. 43-2, p. 760
Life Cycle: Leopard Frog
transformation to
adult nearly complete
adult, three
years old
Sexual reproduction
(gamete formation,
external fertilization)
tadpole
larva (tadpole)
organ
formation
cleavage
eggs
and
sperm
zygote
Fig. 43-3a, p. 760
transformation to
adult nearly complete
adult, three
years old
Sexual reproduction
(gamete formation,
external fertilization)
tadpole
larva (tadpole)
organ
formation
eggs
and
sperm
cleavage
zygote
Stepped Art
Fig. 43-3a, p. 760
Life Cycle: Leopard Frog
blastocoel
gray
crescent
B Here we show the first three divisions of
cleavage, a process that carves up the zygote’s
cytoplasm. In this species, cleavage results in a
blastula, a ball of cells with a fluid-filled cavity.
blastula
C Cleavage is over
when the blastula
forms.
Fig. 43-3b, p. 761
Fig. 43-3b, p. 761
Fig. 43-3c, p. 761
blastocoel
blastula
C Cleavage is over when
the blastula forms.
Fig. 43-3c, p. 761
Fig. 43-3d, p. 761
ectoderm
dorsal lip
future gut
cavity
yolk
plug
neural
plate
D The blastula becomes a three-layered gastrula—a process
called gastrulation. At the dorsal lip, a fold of ectoderm above
the first opening that appears in the blastula, cells migrate
inward and start rearranging themselves.
ectoderm
mesoderm
endoderm
Fig. 43-3d, p. 761
Fig. 43-3e, p. 761
E Organs begin to form as a
primitive gut cavity opens
up. A neural tube, then a
notochord and other organs
form from the primary tissue
layers.
neural
tube
notochord
gut cavity
Fig. 43-3e, p. 761
Fig. 43-3f, p. 761
Tadpole, a swimming larva
with segmented muscles
and a notochord extending
into a tail.
Limbs grow and the tail
is absorbed during
metamorphosis to the
adult form.
Sexually mature, fourlegged adult leopard
frog.
F The frog’s body form changes as it grows and its tissues specialize. The
embryo becomes a tadpole, which metamorphoses into an adult.
Fig. 43-3f, p. 761
Animation: Leopard frog life cycle
43.2 Early Marching Orders
 The location of materials in an egg and
distribution of those materials to descendant cells
affects early development
 Cytoplasmic localization
• In an unfertilized egg, many enzymes, mRNAs,
yolk, and other materials are localized in specific
parts of the cytoplasm
Cytoplasmic Localization
Fig. 43-4a, p. 762
animal pole
pigmented cortex
yolk-rich cytoplasm
vegetal pole
sperm
penetrating
egg
gray
crescent
egg after
fertilization
Fig. 43-4a, p. 762
Fig. 43-4b, p. 762
gray crescent of
salamander zygote
First cleavage
plane; gray
crescent split
equally. The
blastomeres are
separated
experimentally.
Two normal larvae
develop from the
two blastomeres.
B Experiment 1
Fig. 43-4b, p. 762
Fig. 43-4c, p. 762
gray crescent of
salamander zygote
First cleavage
plane; gray
crescent missed
entirely. The
blastomeres are
separated
experimentally.
A ball of
undifferentiated
cells forms.
Only one
normal larva
develops.
C Experiment 2
Fig. 43-4c, p. 762
animal pole
pigmented
cortex
yolk-rich
cytoplasm
vegetal pole
gray crescent of
salamander
zygote
gray crescent
of salamander
zygote
First cleavage
plane; gray crescent
split equally. The
blastomeres are
separated
experimentally.
First cleavage
plane; gray
crescent missed
entirely. The
blastomeres are
separated
experimentally.
sperm
penetrating
egg
gray
crescent
egg after
fertilization
Two normal larvae
develop from the
two blastomeres.
A ball of
Only one
undifferentiated normal larva
cells forms.
develops.
Stepped Art
A
B Experiment 1
C Experiment 2
Fig. 43-4, p. 762
Animation: Cytoplasmic localization
Cleavage Divides Up
the Maternal Cytoplasm
 Cleavage divides a fertilized egg into a number
of small cells but does not increase its original
volume
 The cells (blastomeres) inherit different parcels
of cytoplasm that will make them behave
differently later in development
Two Main Animal Lineages
Differ in Cleavage Patterns
 Protostomes
• Bilateral invertebrates
• Undergo spiral cleavage
 Deuterostomes
• Echinoderms and vertebrates
• Most undergo radial cleavage
• Mammals undergo rotational cleavage
Variations in Cleavage Patterns
a Early protostome embryo.
Its four cells are undergoing
spiral cleavage, oblique to
the anterior–posterior axis:
b Early deuterostome
embryo. Its four cells
are undergoing radial
cleavage, parallel with
and perpendicular to the
anterior–posterior axis:
Fig. 43-5, p. 763
Effects of Yolk Size
on Cleavage Patterns
a Sea urchin egg, with
little yolk. Cleavage is
complete. First cells
formed are equally
sized.
b Frog egg, with
moderate amount
of yolk. Yolk slows
cleavage so lower
cells are larger.
c Fish egg, with a
large amount of
yolk. Cleavage is
restricted to the
layer of cytoplasm
on top of the yolk.
Two cells formed
by first cleavage
mass of yolk
Fig. 43-6, p. 763
Structure of the Blastula
 Blastula
• Cells produced by cleavage
• Structure varies with species’ cleavage pattern
 Blastocyst (mammalian blastula)
• Outer cells secrete fluid into the cavity
• Inner cells, clustered against the cavity wall,
develop into the embryo
43.3 From Blastula to Gastrula
 Gastrulation
• Developmental process during which cells
rearrange themselves into primary tissue layers
 Most animals have three primary tissue layers
• Outermost layer (ectoderm)
• Middle layer (mesoderm)
• Inner layer (endoderm)
Gastrulation in a Fruit Fly
Initiation of Gastrulation
 Gastrulation occurs when certain cells of the
blastula make and release short-range signals
that cause nearby cells to move about, either
singly or as a cohesive group
 Embryonic induction
• The fate of one group of embryonic cells is
affected by its proximity to another group of cells
Experiment: Embryonic Induction
 Transplanted cells of the dorsal lip of the
blastula (descended from the zygote’s gray
crescent) induced gastrulation in salamanders
A Dorsal lip excised
from donor embryo,
grafted to novel site
in another embryo.
B Graft induces a
second site of inward
migration.
C The embryo
develops into a
―double‖ larva, with
two heads, two tails,
and two bodies
joined at the belly.
Fig. 43-8, p. 764
Animation: Embryonic induction
43.4 Specialized
Tissues and Organs Form
 Cell differentiation
• Process by which cell lineages become
specialized
• Lays the groundwork for formation of specialized
tissues and organs
• Based on selective gene expression
 Signaling molecules contribute to differentiation
Morphogens
 Morphogens
• Signaling molecules encoded by master genes
• Diffuse from a source and form a concentration
gradient throughout the embryo
• Have different effects depending on their
concentration in each region
Morphogenesis
 Morphogenesis
•
•
•
•
Process by which tissues and organs form
Some cells migrate to new locations
Sheets of cells change shapes to form organs
Apoptosis shapes body parts such as fingers
 Apoptosis
• Cells die on cue; signals from cells cause other
cells to self-destruct
Morphogenesis: Neural Tube Formation
A Gastrulation
produces a sheet of
ectodermal cells.
B As microtubules
constrict or lengthen
in different cells, the
cells change shape,
and the sheet forms
a neural groove.
C Edges of the
groove meet and
detach from the main
sheet, forming the
neural tube.
neural groove
ectoderm
neural tube
Fig. 43-9, p. 765
Animation: Neural tube formation
Pattern Formation
 Pattern formation
• Process by which body parts form in a specific
place
 Example: Limb bud formation in chicks
• AER at the tips of limb buds induces the
mesoderm beneath to form a limb
Limb Bud Formation in Chicks
mesoderm of
chick embryo
forelimb
A Experiment 1:
Remove wing
bud’s AER
AER
removed
no limb
forms
wing
AER (region of
signal-sending
ectoderm)
B Experiment 2:
Graft a bit of leg
mesoderm under
the AER of a wing
mesoderm
from leg
leg forms
Fig. 43-10, p. 765
Animation: AER transplant
43.5 An Evolutionary
View of Development
 Similarities in developmental pathways among
animals are evidence of common ancestry
 Cytoplasmic localization in the egg induces
expression of localized master genes
 Concentration gradients of master gene
products cause embryonic cells to form tissues
and organs at certain locations
Homeotic Genes
 Positional information established by
concentration gradients of master gene products
affects expression of homeotic genes, which
regulate development of specific body parts
Developmental Constraints
and Modifications
 Physical constraints
• Surface-to-volume ratio
 Architectural constraints
• Existing body frameworks, such as four limbs
 Phyletic constraints
• Master genes determine basic body form
Developmental Constraints
and Modifications
 Mutations that alter the effects of master genes
are often lethal
 Example: Development of somites
• Mesoderm on either side of the neural tube
divides into blocks of cells that will develop into
bones and muscles
Lethal Mutation Affecting Somites
43.1-43.5 Key Concepts
Principles of Animal Embryology
 Animals develop through cleavage, gastrulation,
organ formation, and then growth and tissue
specialization
 Cleavage parcels out material stored in different
parts of the egg cytoplasm into different cells,
thus starting the process of cell specialization
43.6 Overview of Human Development
 Humans begin life as a single cell and go
through a series of developmental stages
• Second week: Blastocyst is embedded in the
mother’s uterus, where it develops
• Embryonic period (first 8 weeks): All organs form
• Fetal period (9 weeks to birth): Organs of the
fetus grow and specialize
• Postnatal growth (after birth): Organ growth and
maturation continues until adulthood
Stages of Human Development
Prenatal and Postnatal Changes
8-week
embryo
12-week
embryo
newborn
2 years
5 years
13 years 22 years
(puberty)
Fig. 43-12, p. 767
43.7 Early Human Development
 Cleavage of a zygote produces a cluster of 16
cells (morula) by the time it reaches the uterus
 By the fifth day, a blastocyst forms, consisting
of an outer layer, a fluid-filled cavity (blastocoel)
and an inner cell mass
• Inner cell mass will form the embryo
• Outer cells will form supportive tissues
Implantation
 Implantation
• The blastocyst ruptures the zona pellucida and
burrows into the lining (endometrium) of the
mother’s uterus
• In ectopic pregnancy, the blastocyst implants
outside the uterus
Extraembryonic Membranes
 The outer layer of the blastocyst gives rise to
four external membranes
• Amnion encloses and protects the embryo in a
fluid-filled cavity
• Yolk sac gives rise to blood and germ cells
• Chorion extends into maternal tissues and
becomes part of the placenta
• Allantois gives rise to blood vessels of placenta
The Placenta
 Placenta
• An organ that functions in exchange of materials
between the bloodstreams of a mother and her
developing child
• Forms from projections of chorion that extend into
blood-filled maternal tissues, and blood vessels of
allantois
Human Extraembryonic Membranes
Early Hormone Production
 Human chorionic gonadotropin (HCG)
• Released by blastula after implantation
• Causes corpus luteum to keep secreting
progesterone and estrogens to maintain the
uterine lining
 The placenta takes over secretion of HCG after
about three months
Fertilization to Implantation
fertilization
in oviduct
implantation
in the uterus
endometrium
Fig. 43-13a, p. 768
endometrial
epithelium
inner cell mass
cavity inside
the uterus
surface layer
cells of the
blastocyst
blastocoel
inner cell mass
Fig. 43-13a, p. 768
Fertilization to Implantation
start of
embryonic
disk
start of
amniotic
cavity
start of
yolk sac
DAYS 10–11. The yolk sac,
embryonic disk, and amniotic
cavity have started to form
from parts of the blastocyst.
actual
size
Fig. 43-13b, p. 769
blood-filled spaces
start of
chorionic cavity
DAY 12. Blood-filled
spaces form in maternal
tissue. The chorionic
cavity starts to form.
actual
size
Fig. 43-13b, p. 769
chorion
chorionic
cavity
chorionic
villi
amniotic
cavity
connective
tissue
yolk sac
DAY 14. A connecting stalk
has formed between the embryonic
disk and chorion. Chorionic villi,
which will be features of a
placenta, start to form.
actual
size
Fig. 43-13b, p. 769
Animation: Cleavage and implantation
Animation: First two weeks of
development
43.8 Emergence of
the Vertebrate Body Plan
 Two weeks after fertilization, the inner cell mass
of a blastocyst is a two layered embryonic disc
 Gastrulation occurs in the third week, forming an
embryo with three germ layers: ectoderm,
mesoderm, and endoderm
• Primitive streak, neural tube and notochord form
• Somites appear on either side of the neural tube
Derivatives of Human Germ Layers
Features of the Embryonic Period
paired neural folds
yolk sac
primitive
embryonic
disk
streak
amniotic
neural groove (below,
cavity
notochord is forming)
chorionic cavity
A DAY 15. A faint band
appears around a
depression along the
axis of the embryonic
disk. This band is the
primitive streak, and
it marks the onset
of gastrulation in
vertebrate embryos.
future brain
pharyngeal
arches
somites
B DAYS 18–23. Organs start to form
through cell divisions, cell migrations,
tissue folding, and other events of
morphogenesis. Neural folds will merge
to form the neural tube. Somites (bumps
of mesoderm) appear near the embryo’s
dorsal surface. They will give rise to most
of the skeleton’s axial portion, skeletal
muscles, and much of the dermis.
C DAYS 24–25. By
now, some embryonic
cells have given rise
to pharyngeal arches.
These will contribute
to the formation of the
face, neck, mouth, nasal
cavities, larynx, and
pharynx.
Fig. 43-14, p. 770
43.6-43.8 Key Concepts
Human Development Begins
 A pregnancy starts with fertilization and
implantation of a blastocyst in the uterus
 After implantation, a three-layered embryo forms
and organ formation begins
 All organs have formed by the end of the eighth
week
43.9 The Function of the Placenta
 Maternal and embryonic blood do not mix
• Vessels of the embryo’s circulatory system
extend through the umbilical cord to the placenta,
where they run through pools of maternal blood
• Substances diffuse across membranes between
maternal and embryonic bloodstreams
 Placental hormones maintain the uterine lining
The Placenta
Fig. 43-15a, p. 771
4 weeks
8 weeks
12 weeks
Fig. 43-15a, p. 771
Fig. 43-15b, p. 771
appearance of the
placenta at full term
umbilical
cord
uterine tissue
amniotic fluid
around fetus
maternal
blood
vessels
fetal
blood vessels
movement
of solutes
to and from
maternal
blood
vessels
(red and blue
arrows)
tissues
of uterus
umbilical cord
blood-filled
space
between villi
chorionic villus
fused amniotic
and chorionic
membranes
Fig. 43-15b, p. 771
43.9 Key Concepts
Function of the Placenta
 The placenta allows substances to diffuse
between bloodstreams of a mother and her
developing child
 It also produces hormones that help sustain the
pregnancy
43.10 Emergence of
Distinctly Human Features
 Embryonic features disappear and the fetus
takes on human appearance about 8th week
 Heartbeat and movements are detected in the
second trimester
 In the third trimester, the brain is formed and
functioning
Development of the Human Embryo
WEEK 4
yolk sac
connecting stalk
embryo
WEEKS 5-6
Fig. 43-16a, p. 772
Fig. 43-16a, p. 772
forebrain
future lens
pharyngeal
arches
developing heart
upper limb bud
somites
neural tube
forming
lower limb
bud
tail
Fig. 43-16a, p. 772
Fig. 43-16a, p. 772
head growth exceeds
growth of other regions
retinal pigment
future external ear
upper limb differentiation
(hand plates develop, then
digital rays of future fingers;
wrist, elbow start forming)
umbilical cord formation
between weeks 4 and 8
(amnion expands, forms
tube that encloses the
connecting stalk and a
duct for blood vessels)
foot plate
Fig. 43-16a, p. 772
Development of the Human Embryo
Fig. 43-16b, p. 773
placenta
Fig. 43-16b, p. 773
Fig. 43-16b, p. 773
WEEK 8
final week of embryonic
period; embryo looks
distinctly human
compared to other
vertebrate embryos
upper and lower limbs well
formed; fingers and then
toes have separated
primordial tissues of
all internal, external
structures now developed
tail has become stubby
Fig. 43-16b, p. 773
Fig. 43-16b, p. 773
WEEK 16
Length: 16 centimeters
(6.4 inches)
Weight: 200 grams
(7 ounces)
WEEK 29
Length: 27.5 centimeters
(11 inches)
Weight: 1,300 grams
(46 ounces)
WEEK 38 (full term)
Length: 50 centimeters
(20 inches)
Weight: 3,400 grams
(7.5 pounds)
During fetal period, length
measurement extends
from crown to heel (for
embryos, it is the longest
measurable dimension, as
from crown to rump).
Fig. 43-16b, p. 773
43.11 Mother as Provider and Protector
 A developing human depends on its mother to
supply the nutrients it requires to grow and develop
• Proteins, carbohydrates, and lipids
• Vitamins and minerals
 Dietary deficiencies affect many developing organs
Teratogens
 The embryo/fetus is also subjected to any toxins
or pathogens to which the mother is exposed
 Teratogens
• Toxic or infectious agents that interfere with
development
• Effects vary with the timing of exposure
Teratogens
 Infectious agents
• Viral diseases (such as rubella), toxoplasmosis
 Alcohol and caffeine
• Fetal alcohol syndrome, miscarriage
 Smoking
• Affects growth and development
 Prescription drugs
• Some medications cause severe birth defects
Fetal Alcohol Syndrome (FAS)
Teratogen Sensitivity
defects in
physiology; physical
abnormalities minor
major
morphological
abnormalities
weeks: 1
2
cleavage,
implantation
3
4
future
heart
future future
brain eye
limb
buds
5
6
7
future
ear
palate
forming
teeth
8
9
16
20–36
38
external genitals
central nervous system
heart
upper limbs
eyes
lower limbs
teeth
palate
external genitals
insensitivity to
teratogens
ear
Fig. 43-17, p. 774
43.10-43.11 Key Concepts
Later Human Development
 By the time the fetal period begins, the
developing individual appears distinctly human
 Harmful substances that get into a mother’s
blood can cross the placenta and cause birth
defects in the developing embryo or fetus
43.12 Birth and Lactation
 Labor is the process of giving birth
• Amnion ruptures, cervix dilates
• Contractions force the fetus, and later the
placenta (afterbirth), through the birth canal
 Oxytocin stimulates muscle contractions in a
positive feedback loop during birth
• Secreted by the posterior pituitary
Birth and Afterbirth
Fig. 43-19a, p. 776
placenta
wall of uterus
umbilical cord
dilating cervix
Fig. 43-19a, p. 776
Fig. 43-19b, p. 776
Fig. 43-19c, p. 776
placenta detaching
from wall of uterus
umbilical cord
Fig. 43-19c, p. 776
Animation: Birth
Nourishing the Newborn
 Newborn humans are nourished with milk
secreted by the mother’s mammary glands
 Hormonal control of lactation (milk production)
• Prolactin, secreted by the anterior pituitary,
triggers milk synthesis
• Declines in progesterone and estrogen
production after birth increase milk production
• Oxytocin stimulates release of milk into milk ducts
Lactation and Mammary Glands
nipple
adipose
tissue
milk-producing
mammary gland
milk duct
Fig. 43-20, p. 776
43.12 Key Concepts
Birth and Lactation
 Positive feedback control plays a role in the
process of labor, or childbirth
 After birth, the newborn is nourished by milk
secreted by mammary glands
Animation: Anatomy of the breast
Animation: Blastomere separation I
Animation: Blastomere separation II
Animation: Fetal development
Animation: Formation of gray crescent
Animation: Proportional changes during
development
Animation: Sensitivity to teratogens
Animation: Stages of development
Animation: Structure of the placenta
Animation: Three variations in
gastrulation
Animation: Weeks 3 to 4 of development
ABC video: Mermaid Baby
ABC video: Bonus for a Baby
Video: Mind-boggling births